Interferons are one of the body's natural defensive responses to foreign components such as microbes, tumors, and antigens. Interferons have potent antiproliferative and immune-regulatory activity. Interferon gamma (IFN-gamma), one of three major classes of interferons, is a potent anti-viral and anti-microbial substance produced by certain white blood cells. One of the most efficient producers of interferon gamma is a natural killer (NK) cell. NK cells are activated during infections by viruses and by other intracellular pathogens, such as parasites and bacteria. Soluble substances, such as interleukins, produced by infected cells activate NK cells to secrete interferon gamma. Injection of interleukins into patients to stimulate NK cells has not been a successful therapeutic approach because of the toxicity involved.